1. Field of the Invention
The present invention relates to a melt extrusion process for making multilayer articles, particularly multilayer articles having electrical or ionic conductivity. The articles are suitable for use in electrochemical applications, such as batteries, fuel cells, electrolysis cells, ion exchange membranes, sensors, electrochemical capacitors, electrochromic windows, and modified electrodes. Of particular interest is the use in lithium-ion batteries.
2. Description of the Related Art
A battery can be made of one or more cells, usually connected to achieve a particular voltage and/or capacity. A cell includes three major components: the positive electrode, the negative electrode, and an electrolyte. A porous, polyolefin-based separator is also commonly present to prevent electrical contact between the two electrodes. In practice, a cell often also contains anode and cathode current collectors which are conductive layers, usually metallic, each of which will have tabs for external connection. A battery often also requires a package which may contain several individual cells and out of which the tabs will protrude.
Lithium-ion polymer battery cells utilize electrode elements comprising flexible sheets of polymeric composition in which are dispersed finely-divided particulate materials capable of reversibly intercalating lithium ions during battery charge/discharge cycles. The polymeric materials in the electrodes and the separator element are generally the same or similar to provide adhesion between the layers in the cell.
Solvent coating is often employed to produce the individual layers for a lithium-ion polymer battery, followed by lamination of the layers to form cells. However, solvent coating has process limitations resulting in defects such as bubbles due to high drying rates; pinholes due to entrapped air or surface tension defects; settling of suspended solids during solution storage or handling; flooding and floating of suspended solids during drying; gelation of thermodynamically unstable solutions; and cracking due to shrinkage stresses during drying.
Melt extrusion processes to form multilayer articles from polymeric materials are known in the art. They include co-extrusion processes in which multiple feed streams are extruded through a single-slot die. In one kind of co-extrusion process, multiple feed streams are introduced into a feed block and then into the die. In a second type of co-extrusion process, the multiple streams are introduced into a multi-manifold, single-slot die. Another melt extrusion process is a multilayer extrusion coating process in which the feed streams are introduced into a multi-manifold multi-slot die.
Melt processes, particularly melt extrusion processes, offer lower investment and higher capital productivity as compared with solvent coating processes. Multilayer extrusion processes offer the additional advantage of fewer process steps and further increased productivity.
On the other hand, a disadvantage to extrusion, as practiced in the existing art, is the requirement of melting the polymer at relatively high temperatures, making it difficult to process any volatile liquids or thermally unstable materials. Another disadvantage to extrusion as practiced in the existing art, is that mixing is carried out in the molten state; imposing high shear stresses on the mixtures. This can cause degradation of some of the desired properties of the composition.
Therefore, there is a need for an improved multilayer extrusion process which is suitable for materials sensitive to heat and/or shear stress.
The present invention relates to a multilayer extrusion process that minimizes exposure of the components to shear and elevated temperatures. The process includes a combination step in which the components are combined to form a macroscopically homogeneous mass. This is done separately for the materials in each layer. Each of the macroscopically homogeneous masses are then fused to form plastically formable masses. At least two plastically formable masses are simultaneously forced through a die containing at least one slot onto a support to form a multilayer structure. The support can be a permanent support or a temporary support.
In a preferred embodiment of the invention, the die is a multislot die having multiple input manifolds and multiple slots, and the plastically formable material from each extruder is extruded into a separate manifold of the die.
In a second preferred embodiment of the invention, the composition of at least one of the layers comprises an electrochemically active material, an electrically conductive material, and an ionically conductive mixture.